Vehicular suspension systems usually employ some type of compression spring assembly for cushioning shocks during operation of the vehicle. Because compression springs generally exhibit a constant rate of compression or constant spring rate, they may be incapable of fully absorbing rapidly changing shock loads. As a result, an undesirable shock level may be transmitted through the suspension system to the frame of the vehicle.
Also, many vehicles such as pick-up trucks, vans, campers, school buses, and the like are operated as dualpurpose vehicles. Sometimes they carry only a driver and possibly a passenger or two; at other times they will be fully loaded and need proper overload spring capacity. The fully-loaded spring capacity is obviously more than the desirable amount for the minimum load condition.
In an effort to provide a more satisfactory cushioning response, a plurality of compression spring assemblies, each exhibiting a different spring rate, have been positioned to jointly cushion loads. While this type of system may provide a cushioning response to a greater variety of shocks, it will remain limited in performance as long as the individual springs are capable of only providing a constant rate of compression.
As a further improvement, known suspension systems have attempted to modify one of the spring assemblies to provide a variable spring rate capable of more fully cushioning changing shock loads.
One of the more desirable systems to cope with this problem is that which uses automatically-variable pressure bags. These air bags operate over a wide pressure range, but their satisfactory operation depends on a suitable air source that must be replenished constantly as the vehicle is driven because the automatic leveling valves constantly function in attempting to maintain the pre-set level height of the vehicle by letting air in and out as the suspension system moves up and down in response to the irregular road surface. They also operate at relatively high pressure during maximum load operation; therefore, they sometimes blow out like a tire with age. They can also be cut by a foreign object and blow out. In addition to these possible problems, the leveling valves are subject to maintenance at various intervals or the air bag may over inflate or under inflate and go flat. Either case may render the vehicle inoperative.
An additional problem encountered with the use of the air bag only, is the mismatch of the vehicle shock absorber. For example, when the vehicle is lightly loaded, a relatively light shock absorber is desirable in order to enjoy the benefits of the soft air cushion provided by the low pressure in the air bag. A light or relatively moderate shock absorber will provide a comfortable, smooth ride when light loads are carried, but cannot control the ride or rebound of the springs when the heavy loads are carried with high pressure in the air bags. With an air bag suspension system operating in a conventional manner, the shock absorber can only be chosen for one of the conditions and will be compromised for the other. This can result in a very unsatisfactory operational condition somewhere in the load range of the vehicle.
A preferred embodiment of applicant's invention overcomes the problems of the known devices of a similar type, while still providing the known benefits of using an air-bag assembly.
In accordance with the preferred embodiment of the present invention, a novel suspension system comprises an air-bag assembly acting in cooperation with a spring assembly comprising a plurality of sequentially deformable elastomeric members. The air-bag assembly may, in one embodiment, be pre-inflated to a desired level and function to absorb a portion of the shock load with the balance of the load being borne by the elastomeric members as they are sequentially deformed. Because the elastomeric members are sequentially deformable, they exhibit a variable spring rate which functions to absorb the remaining portion of the shock load which would have been transmitted through the air-bag assembly.
Applicant's preferred embodiment dispenses with the complex high-pressure valve apparatus necessary for varying the air pressure within the air-bag assembly. Rather, compressed air at a lower pressure may be selectively supplied from a conventional source, such as an air pump, which may be removably attached to an intake nozzle extending from the air-bag assembly.
Conveniently, a further embodiment of applicant's invention may provide an air-bag assembly which functions, in cooperation with some of the elastomeric members which are initially deformed by a given load, as a primary cushioning system, and with others of the elastomeric members which only become deformed by a heavy load providing a secondary spring reacting to and cushioning intense shock loads. An advantage of this embodiment resides in the fact that the elastomeric members are initially undeformed, or only relatively slightly deformed, by the weight of the vehicle, thereby avoiding a permanent set from occurring in the elastomeric material. Furthermore, an accurate tuning of the suspension system force curve can be readily accomplished by substituting elastomeric members of differing sizes, or changing the number of elastomeric members within the assembly, or both.
Some of the elastomeric members are intended to be compressed only during an overload condition. A light load or soft-ride condition may be provided by a conventional, externally-mounted soft-ride shock absorber positioned between the frame and axle assembly of the vehicle and acting in cooperation with the more easily deformable elastomeric members to give a soft, smooth ride when the vehicle is lightly loaded. Preferably, a further shock absorber is incorporated within the device of the invention and has stiffer characteristics and acts to provide the desired ride control in cooperation with those elastomeric members which are compressed only in response to heavy loads, such as occurs when the vehicle is heavily loaded or encounters greater-than-normal compression from extreme driving conditions.
The combination of applicant's novel suspension system and a conventional shock absorber allows both a soft-ride range and a firm-ride range to be easily tailored to suit a particular vehicle's requirements. Furthermore, the soft-ride, externally mounted shock absorber cushions a majority of the light shocks encountered and is easily replaced at minimal cost, whereas the novel suspension assembly comprising the preferred embodiment receives considerably less wear and hence need not be replaced as frequently. This results in a savings on maintenance cost over a long period of usage.
A significant benefit of the preferred embodiment resides in the ability to manufacture a commercial vehicle, such as a school but with weight-saving materials and techniques that would otherwise be short-lived if the extremely hard conventional truck springs were used in the suspension system. The rough ride of conventional truck-type springs has proven, more than any other factor, to be responsible for the need to frequently replace school buses that are literally shaken to destruction before the running gear is ready for retirement. Even though conventional air-ride systems are used on city buses, they often require frequent maintenance due to their elaborate construction. In comparison, the preferred embodiment provides the desirable soft ride with the low maintenance of conventional spring assemblies.
In a further embodiment of applicant's invention, an air-bag assembly may be partially vacuumized through a passageway joining a source of sub-atmospheric pressure, such as the vehicles's engine manifold, with the partial vacuum within the air-bag assembly functioning to resist and cushion shock loads. As in the previous embodiments, it would be within the scope of the present invention to partially vacuumize the air-bag to a desired level, with the air-bag and sequentially deformable elastomeric members jointly cushioning the shock loads.
Conveniently, a further embodiment of the present invention may employ only a plurality of sequentially actuable elastomeric members to provide a spring assembly exhibiting a varying spring rate in compression. Such an assembly should not be limited to use in a suspension system, but may be adaptable for use wherever a variable rate spring assembly is employed. Thus, the present invention provides a variable rate overload capacity spring that provides a smooth ride at low vehicle weights but also provides adequately damped rebound control at the heavy weights.
The suspension system of this invention does not operate at extreme high pressure as with the prior art air-bag because the overload is carried substantially entirely by the elastomeric members, not high pressure air. Thus, the air-bags are considerably less likely to have a blow-out with age. The low pressure bag is also less likely to be damaged by a foreign object. Also, the system of this invention does not need an expensive air system to replenish the air while driving. If it is desired to provide the system with a level-ride feature, this can be controlled by low pressure vacuum off the engine which is always available if the vehicle is capable of running.
A further advantage is that the normal function produced by the expansion and contraction of the elastomeric members produces a natural rebound restraint. Therefore, a wide selection of materials may be used to produce varied amounts of natural rebound control, making it possible to provide a low cost unit for many applications where the precise control of an automobile is not necessary.